1. Field of Embodiments
The present embodiments relate generally to orthopedic devices. More specifically, the present embodiments relate to screws for orthopedic devices that are typically placed within or adjacent to vertebrae (e.g., pedicles). It should be noted, however, that the screws disclosed herein may also be used in other orthopedic applications. For example, the screws may be used for repositioning fractured bones relative to one another, or used in an external fixator for trauma patients.
2. Description of Related Art
During spinal fusion surgery, bone screws may be fixed to adjacent vertebrae and interconnected with rods that span the screws to stabilize the spine during the healing process. The screw heads typically include a slot that is intended to receive a rod. The rod may be at least partially secured within the slot by a cap or set screw. The rods interconnecting the bone screws may span one or more vertebral levels (e.g., one to two vertebral levels for a spinal fusion of a degenerative condition, eight to twelve vertebral levels for scoliosis, etc.).
During an orthopedic surgical procedure, a number of factors can impact bone screw placement and alignment. These factors include, but are not limited to, spinal curvatures, variations in patient anatomy, and small imprecisions in screw placement by the surgeon. It is generally desirable for bone screws to be configured to be adjustable in order to achieve a desired alignment. However, with conventional bone screws, greater adjustability of the bone screw typically means the screw is relatively bulky (e.g., relative to its less adjustable counterparts). Bulky bone screws can cause pain and irritation to some patients.
Bone screws can be classified as monoaxial, polyaxial or uniplanar based on their adjustability.
Monoaxial bone screws are the most simplistic and not particularly useful for accommodating screw placement and alignment. The screw head of a monoaxial screw allows adjustment to the rod only in the direction perpendicular to the longitudinal axis of the screw. When the rod is secured in the slot of the bone screw, the longitudinal axis of the rod is substantially perpendicular to (i.e., at a 90° angle to) to the longitudinal axis of the bone screw.
Polyaxial screws are commonly used to overcome the variations in screw placement and alignment. The relationship between the screw axis and the rod axis can be variable but still be locked solidly in place (e.g., the screw head of a polyaxial screw may be configured to swivel approximately 20° off the screw axis). This adjustability allows rods to be connected to multiple screws that may be placed medial or lateral to one another and to provide for lordotic and kyphotic spinal alignments. Conventional polyaxial screws are larger and bulkier than monoaxial screws (e.g., because of the structures that provide for the screw head of a polyaxial screw to swivel).
Uniplanar screws have screw heads that may deviate from the screw axis but only in the plane of the slot for the rod; they typically do not adjust to medial or lateral rod positions (like polyaxial screws). This type of screw is more commonly used in scoliosis surgery where there may be a degree of cranial or caudal angulation (such as the sagittal plane of the spine), but there is little medial lateral screw placement deviation and the surgeon additionally needs rigid control of the screw to manipulate it in the coronal and axial plane of the spine. Similar to polyaxial screws, conventional uniplanar screws are large and bulky because of the configuration of their single plane swivel mechanism.
Because many scoliosis patients are children or smaller-sized adults and because many scoliosis fusions are performed in the kyphotic thoracic spine with less soft tissue coverage, the larger, bulkier polyaxial and uniplanar screws are often prominent and may cause pain and irritation to the patient. In some cases, the pain and irritation may rise to the level where a second surgery to remove the screws and rods is necessary.
A need exists for improved uniplanar bone screws, including bone screws that may address one or more of the above described disadvantages.